Add a README.

README.md

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+# enter-uml
+
+An unprivileged sudo.
+
+`enter-uml` gives you root privileges to interact with a uml kernel of
+your choosing.  You may test system calls, filesystems, and some device
+drivers without crashing your machine and without root privileges.  This
+is also useful if you have root but module loading is restricted.
+
+`enter-uml` does some minimal set up so that your filesystems are reused
+— you do not need a root filesystem image — and so that the inner
+context is very close to the outer environment.  This makes usage
+similar to sudo; your privileges have been raised and you retain access
+to the same filesystem environment to run commands from.  Limitations:
+there is no network access, and dynamic device node creation is done by
+launching udev/eudev by hand.
+
+A suitable kernel can be installed with
+
+    sudo apt-get install user-mode-linux
+
+on Ubuntu and Debian.  You can also download from
+<http://uml.devloop.org.uk/kernels.html> or build your own.  If you want
+to step inside the kernel, you can use the `--gdb` flag and set
+breakpoints before entering.
+
+`enter-uml` requires Python 3.3.
+